1. Field of the Invention
This invention relates to electronic packaging and more particularly to an improved circuit board and an improved method of making circuit boards.
2. Background Art
There is a great deal of prior art directed to printed circuit boards and to processes for making printed circuit boards. The technology for making large high density printed circuit boards using plating techniques is well known. For example, see the introduction of U.S. Pat. No. 4,080,513 for a general summary of prior art in this area.
One of the problems encountered when fabricating high density circuit boards is the problem of precisely locating key features of the board after the board has passed through several steps in the fabrication process. For example, the location of circuit lands must be precisely known so that subsequent drilling steps can pass through these lands. Dimensional instability is a particularly severe problem when fabricating multilayer circuit boards which go through a series of laminating cycles. For example, U.S. Pat. No. 3,969,177 describes a technique for addressing this problem area. However, as the density and size of the boards being fabricated increases, the need for greater dimensional stability increases so that holes which are subsequently drilled in the board will intersect key features of the board such as lands in the circuit pattern thereby effecting an electrical connection to these features. If the board is not dimensionally stable, these drilled holes may miss the conductive pattern and the board will not function properly.
Most high density circuit boards use epoxy resin laminates which are reinforced with glass cloth. The glass cloth is included in order to provide strength. However, when the epoxy is softened as occurs during subsequent operations such as lamination, the glass cloth tends to move in an irregular and uncontrolled manner, thus the key features in the circuit pattern tend to move in an uncontrolled manner and these features can not be precisely located for subsequent drilling operations.
The technique described in U.S. Pat. No. 4,030,190 (Varker) seeks to eliminate this problem by using a relatively thick sheet of copper to impart dimensional stability instead of relying on the properties of the epoxy glass substrate for dimensional stability.
U.S. Pat. No. 4,030,190 (Varker) describes a sequential process for making multilayer circuit boards wherein dimensional stability is achieved during the manufacturing operation by building up the various layers of the board onto a substrate or base plane of copper which has a high degree of dimensional stability. A high density circuit board manufactured using this sequential technique is described in an article by D. P. Seraphim, entitled "A New Set of Printed Circuit Technologies for the IBM 3081 Processor", published in the IBM Journal of Research and Development, Vol. 26, No. 1, January 1982.
An alternative technique for manufacturing circuit boards involves laying down thin insulated wires on a substrate which has been coated with an adhesive. Holes are drilled into the board, cutting the wires at the places where connections are desired. These holes are plated, thereby making connections to the wires. Boards made by this process are generally termed encapsulated wire boards. One commercial application of this technology is known by the trade name "Multiwire" (trademark owned by Kollmorgen Corp). This type of technology is, for example, shown in U.S. Pat. Nos. 4,097,684 (Burr); 3,646,572 (Burr); 3,674,914 (Burr) and 3,674,602 (Keough).